PSV: Calculate mass (mole) fractions from mole (mass) fractions for a mixture

Compositions and Mixtures

Since we are chemical engineers, every now and then we actually deal with chemicals!

In this lecture we will remind you of a bunch of things that you should have learned in chemistry about compositions and chemical "amounts".


Atomic weight is a measure of the mass of an atom (C12 -> 12 units)


Molecular weight is the sum of the atomic weights of the atoms that make up a compound.


A mole (or gram-mol) is the amount of a species (atom or molecule) whose mass is numerically equivalent to its molecular weight. (For example 12 grams of C12 = 1 gram-mol of C12)


Molecular (atomic) weight can be used as a conversion factor between the number of X-moles and the mass in X units!


How do you convert 1 lb-mol of H2O to gram-moles of H2O?

Compositions as fractions:



Percent = parts per hundred. We may also use parts per million (ppm) or parts per billion (ppb). (Purely for simplicity in describing dilute solutions, so that we need not say 0.0001% or 1.0e-7%)


Calculate mass (mole) fractions from mole (mass) fractions for a mixture


On a humid day, air may have a composition like this: O2: 19%, N2: 72%, and H2O: 9.0% (all percents are by MASS). What is the composition in mole fractions?

O2 N2 H2O

Compositions as concentrations:


Concentration refers to the "amount" (measured in mass or moles) of a material per unit volume (much like the density: in fact, mass concentration <-> density!).


The word molarity typically refers to the molar concentration in solution